Strictly speaking, “reductive amination” refers to the reaction of an aldehyde or ketone with ammonia (or a primary or secondary amine) and hydrogen in the presence of a metallic hydrogenation catalyst to give a primary, secondary, or tertiary amine product. Primary and secondary alcohols also undergo the same reaction, except that hydrogen is not consumed in the reaction. It has been found in general that catalysts useful in reductive amination of aldehydes and ketones are also useful in the amination of alcohols, though the reduction of an alcohol in general requires considerably higher temperature.
Catalysts useful in reductive amination and alcohol amination processes have been the subject of a large volume of work by chemists, and the prior art is replete with patents concerning catalytic materials and/or processes using catalytic materials as the including the following U.S. Pat. Nos. 6,159,894; 6,057,442; 6,037,295; 6,046,359; 5,986,138; 5,958,825; 5,723,641; 5,367,112; and 4,152,353, as well as PCT International Applications WO 96/01146 and WO 94/24091. All patents and patent application publications mentioned herein are incorporated by reference thereto in their entirety.
Catalysts useful in reductive amination have often historically comprised metals such as Ni, Co, and Cu as the active component, and are sometimes referred to as hydrogenation/dehydrogenation catalysts because they are active in both types of reactions. Other elements from the Periodic Table of the Elements are frequently incorporated into the catalyst to optimally tailor the activity or selectivity of the catalyst for the particular process in which it is employed. U.S. Pat. Nos. 4,153,581; 4,409,399; 4,152,353 are descriptive of some of the more successful types of reductive amination catalysts. Habermann, in U.S. Pat. No. 4,153,581, discloses a method of preparing amines using a catalyst comprising from about 20 to about 90 percent cobalt, from about 8 to about 72 percent copper, and from about 1 to about 16 percent of a third component selected from the group consisting of iron, zinc, zirconium, and mixtures thereof. The catalyst of U.S. Pat. No. 4,153,581 is specified to comprise at least about 20 percent cobalt. Since cobalt is a relatively expensive metal, it is desirable for practical reasons to have at hand a catalyst useful in the reductive amination of alcohols, etc., which has equal or superior activity to cobalt-bearing catalysts at a reduced cost over the cobalt-bearing catalysts.
Reductive amination process conditions are typically used to make primary amines by reaction of an alcohol with ammonia. Good selectivity to the primary amine is usually achievable when reacting a secondary alcohol is the presence of excess ammonia over a suitable catalyst and under reaction conditions known to those skilled in the art. Primary alcohols as reactant, however, under the same conditions and catalyst give rise to lower primary amine selectivity, in favor of significantly higher secondary amine product and significantly higher undesirable “hydrogenolysis” by-products, especially at higher levels of alcohol conversion. The hydrogenolysis by-products are formed by reductive cleavage, or the formal addition of hydrogen across C—C, C—O, and C—N bonds.
In the case of the amination of diethylene glycol, the primary commercially-useful products are 2-aminoethoxyethanol, morpholine, and bis(aminoethyl) ether. By-products formed by hydrogenolysis reactions and related aminated hydrogenolysis by-products include: methane, carbon dioxide, ethylene glycol, ethanol, ethylamine, ethanolamine, ethylenediamine, 2-methoxyethanol, and 2-methoxyethylamine. Higher molecular weight by-products such as N-ethylmorpholine, N-aminoethylmorpholine, 2-(N-ethylaminoethoxy)ethanol, etc., are also formed under reaction conditions. The formation of these materials leads to lower yields of the desired products, and also complicates the purification process. A catalyst that yields less of these by-products attendant to the production of desired molecules is advantageous from a commercial perspective.
U.S. Pat. No. 4,152,353 discloses catalysts containing Ni (20–49%), Cu (36–79%), and Fe, Zn, and/or Zr (1–15%) useful in the conversion of alcohols to primary amines. U.S. Pat. No. 6,057,442 described catalysts containing Ni (14–70% as NiO), Cu (1–30% as CuO), and Zr (20–85% as ZrO2), with Al2O3 and/or MnO2 (0–10%), useful in the conversion of alcohols to amines. Examples in this patent give results in the amination of diethylene glycol to 2-aminoethoxyethanol and morpholine. However, the examples in these patents do not specify the selectivities to hydrogenolysis by-products. However, in our experience similar catalysts afford relatively high levels of hydrogenolysis by-products in the amination of diethylene glycol. In practice, the high levels of these by-products present yields inferior products, purification problems, and lower overall yields of the desired amine products. Thus, if catalysts with improved selectivities to the desired primary amines over prior art reductive amination catalysts were provided, such catalysts would represent a significant advance in the art, commensurate with the degree of reduction of by-product formation during their use and the cost to manufacture the catalyst.